It has long been known in the investigation of subterranean earth formations traversed by a borehole that measurements or "logs" of acoustic energy introduced into the formation can yield extremely useful information about various formation parameters and characteristics. Accordingly, it is conventional to introduce a logging tool into a borehole containing some form of acoustic wave transmitter and receiver, to direct acoustic energy from the transmitter into the formation adjacent the borehole elevation of interest, and thereafter to record with the receiver the resultant acoustic waves returning from the formation. Some types of conventional logging tools include more than one acoustic transmitter and more than one acoustic receiver.
Parameters useful in the evaluation of oil and gas reservoirs, other mineral deposits and certain engineering problems may be derived from the amplitudes of acoustic well logging data. However, there are differences in the individual transmitter and receiver sensitivities of any acoustic logging tool. If one attempts to make relative amplitude estimates by comparing values associated with different transmitters and receivers, the estimates will be adversely influenced by these sensitivity differences. Furthermore, with conventional technology, it does not appear that the transmitter (or transmitters) and receiver (or receivers) of an acoustic logging tool can be constructed so as to maintain the required amplitude sensitivity tolerances over the duration of a normal logging run. What is needed is a method for deriving necessary corrections from the amplitude data themselves.
Prior art techniques do not solve the problem of how substantially to eliminate the problem of variation in both transmitter and receiver sensitivity. For example, the multiple acoustic receiver and transmitter system described in U.S. Pat. No. 3,524,162, issued Aug. 11, 1970 to Zill, employs an acoustic logging tool having two transmitters (T and t), and two or more receivers located between the transmitters. Zill teaches that four signals should be recorded, each received at one of a pair of receivers (R.sub.1 and R.sub.2) in response to transmission of acoustic energy from one of the transmitters (T or t). Zill assumes that the signals emitted by the two transmitters have identical amplitude, and then proceeds to correct the data for the effects of the transmission factors (h.sub.a, h.sub.b, h.sub.c, and h.sub.d) corresponding to the lateral portion (i.e. the portion perpendicular to the well axis) of the acoustic wave path of each recorded signal, and for the transfer coefficients (g.sub.1 and g.sub.2) of the receivers, by multiplying together the two amplitude ratios formed from each pair of recorded signals associated with the same transmitters. Zill does not suggest how to correct for differences in sensitivity (or other characteristics) between the two transmitters employed.
As described in U.S. Pat. No. 3,302,166, issued Jan. 31, 1967 to Zemanek, it is conventional to employ a tool with a transmitter positioned between two pairs of receivers for logging the travel time of acoustic logging signals as follows. The transmitter is fired and the resulting acoustic arrival recorded at one pair of receivers (the "first" pair). Then the tool is moved to a new location in the well such that the other pair of receivers (the "second" pair) spans the same portion of the formation originally spanned by the first pair of receivers. Then, the transmitter is fired again and the resulting acoustic arrival is recorded at the second pair of receivers. These two pairs of recorded signals are used to correct the data for the effect of unequal distances traveled through the borehole fluid between different ones of the recorded arrivals. Similarly, U.S. Pat. No. 4,460,986, issued July 17, 1984 to Millouet, et al., discloses a tool having one or more acoustic transmitters and two or more acoustic receivers (or two or more transmitters and one or more receivers) and teaches a logging method wherein the tool is moved relative to the well between various events of transmission and recording of signals at the receivers. However, neither U.S. Pat. No 3,302,166 nor U.S. Pat. No. 4,460,986 suggests any technique for correcting both for variations in transmitter sensitivity and for variations in receiver sensitivity.